Very Computer

In /bin/sh, I tried to redirect the standard error and standard output to
/dev/null.

I still has the error message from the following command:

        ls /xyz 2>&1 > /dev/null

but not from the following command:

        ls /xyz 2>&- > /dev/null

I thought >&1 is the same as >&-. Any comments are appreciated.

Thank you.

Larry

'>&-' closes stdout.  Given

$ ls /etc/motd /etc/nonesuch
/etc/nonesuch not found
/etc/motd

you can throw away stderr by redirecting it to /dev/null,

$ ls /etc/motd /etc/nonesuch 2>/dev/null
/etc/motd

or throw away stdout by redirecting it to /dev/null,

$ ls /etc/motd /etc/nonesuch >/dev/null
/etc/nonesuch not found

or throw away both by redirecting stdout to /dev/null, and dup'ing
stderr to stdout.

$ ls /etc/motd /etc/nonesuch >/dev/null 2>&1

Note the difference between that command and

$ ls /etc/motd /etc/nonesuch 2>&1 >/dev/null
/etc/nonesuch not found

which dups stderr to stdout (probably the terminal), and then throws
away stdout.
--
Jim Davis               | "You mean about your close personal relationship

:
: In /bin/sh, I tried to redirect the standard error and standard output to
: /dev/null.
:
: I still has the error message from the following command:
:
:       ls /xyz 2>&1 > /dev/null

Order counts here.  If you redirect standard error to the same place as
standard output and *then* redirect standard output then standard error
won't follow it.  At the time you redirect standard error, standard
output is still pointed at the terminal (presumably).  When you later
redirect standard output to /dev/null standard error is left pointing
to the terminal.

What you intended, I think, is:

     ls /xyz > /dev/null 2>&1

Which redirects standard output to /dev/null and *then* sends standard
error after it.

: but not from the following command:
:      
:       ls /xyz 2>&- > /dev/null
:
: I thought >&1 is the same as >&-. Any comments are appreciated.

No, >&- closes the descriptor.  This command directs standard output to
/dev/null and closes standard error completely.  It has much the same
effect (no output or error messages) but is logically completely
different.
--
Frank Peters  -  UNIX Systems Programmer  -  Mississippi State University

RTFM.

(a) Redirections take effect from left to right, thus:

        foo 2>&1 >/dev/null

makes stderr a copy of stdout (which probably writes to the tty)
and then redirects stdout to /dev/null, whereas

        foo >/dev/null 2>&1

redirects stdout to /dev/null, and the makes stderr a copy of stdout
(so that stderr also writes to /dev/null).

(b) X>&- means close file descriptor X, whereas X>&1 means make file
descriptor X a copy of file descriptor 1 (stdout).

That should be:

        ls /xyz >/dev/null 2>&1

From the sh(1) man page:

     The order in which redirections are  specified  is  signifi-
     cant.   The shell evaluates redirections left-to-right.

Therefore, the problem with your version is that it first copies file
descriptor 1 (stdout) to fd 2 (stderr), and then changes fd 1.  Fd 2 is
still referring to the file that fd 1 originally referred to (the
terminal).

Quote:>but not from the following command:

>    ls /xyz 2>&- > /dev/null

>I thought >&1 is the same as >&-. Any comments are appreciated.

">&-" closes the specified file descriptor, rather than copying another
file descriptor.  There's a difference between a closed file and an open
file connected to /dev/null.
--
Barry Margolin
System Manager, Thinking Machines Corp.

What is that difference?

---Tom

Writes to a closed file descriptor fail, as do dup2() and the various
fcntl() calls.  Writes to a file descriptor open to /dev/null simply
disappear, and all the other calls work.  It's the difference between
valid and invalid file descriptors.

Chet

--
        ``Times are hard, and you're afraid to pay the fee.
          So you find yourself somebody who can do the job for free.''